The present invention relates to a magnetic bubble memory device and in particular to a high density magnetic bubble memory device suited to be used as a memory of large capacity.
In the field of the magnetic bubble memory devices, there has been proposed a bubble memory device having a composite structure in which a transfer path formed through ion implantation (hereinafter referred to as ion implantation transfer path) and a transfer path made of a permalloy (hereinafter referred to as permalloy transfer path) are employed to attain a high density and high integration degree. See Japanese Patent application Laid-Open No. 57-40791 laid open on Mar. 6, 1982 (Japanese Patent application No. 55-116675 filed on Aug. 5, 1980). The ion implantation transfer path which is easy to be realized at an increased density constitutes a region serving for storing information, while the functional parts serving for writing, reading and the like operation are constituted by the permalloy transfer path.
FIG. 1 schematically shows an example of the conventional magnetic bubble memory device of the composite structure in which a permalloy transfer path is provided in vicinity of an ion implantation transfer path. In FIG. 1, reference numeral 4 denotes a non-magnetic garnet substrate, and 5 denotes a magnetic garnet layer formed on the substrate 4 and capable of holding and propagating magnetic bubbles. In a surface of the magnetic garnet layer 5, ions such as H.sup.+, H.sub.2.sup.+, Ne.sup.+ or the like are implanted with a predetermined ion implantation pattern. Reference numeral 21 denotes ion-implanted regions, and 22 denotes a non-implanted region. A cylindrical magnetic domain or magnetic bubble 1 is shown to exist under a biasing magnetic field H.sub.B of a predetermined magnitude applied in the direction shown in FIG. 1. An arrow 10 indicates the direction of magnetization within the bubble domain and arrows 11 indicate the direction of magnetization outside of the bubble domain. The ion-implanted region 21 exhibits inplane magnetization (as shown in arrows 12) due to magnetostriction produced upon ion implantation. At the boundary 2 between the ion-implanted region 21 and the non-implanted region 22, i.e. the edge of the ion-implanted region 21, there is developed an attractive (or converging) charged wall of positive charge attracting the magnetic bubble 1 or a repulsive (or diverging) charged wall of negative charge repulsing the magnetic bubble 1, in accordance with a driving magnetic field rotating in the plane of the magnetic garnet layer 5. The position of the attractive charged wall is moved along the edge (hereinafter referred to as ion implantation transfer path) of the ion-implanted region 21 in dependence on the direction of the in-plane rotating field to transfer the magnetic bubble. A permalloy transfer path 3 is provided on the magnetic garnet layer 5 through an insulating film 6 of SiO.sub.2 or the like. The magnetic bubble 1 moving along the boundary 2 is transferred across the junction with the permalloy transfer path 3 to the latter under attraction exerted by a magnetic pole produced at an end portion of the permalloy pattern. However, in the conventional magnetic bubble memory device, difficulty has been encountered in effecting smooth transfer of the magnetic bubble from the ion implantation transfer path to the permalloy transfer path. More particulary, since the ion implantation transfer path is realized by making use of the boundary 2 between the ion-implanted region 21 and the non-implanted region 22, the magnetic bubble is required to traverse a portion 2' of the boundary 2.
FIG. 2 shows a cross section taken along lines II--II of FIG. 1. In the ion implantation transfer path, the magnetic bubble 1 is present below the ion-implanted region 21 and moved along the boundary 2 between the ion-implanted region 21 and the non-implanted region 22. Accordingly, upon transfer of the magnetic bubble 1 from the ion-implantation transfer path to the permalloy transfer path, the magnetic bubble 1 is required to traverse the boundary portion 2' at the junction between the ion implantation transfer path and the permalloy transfer path to enter the non-implanted region 22 underlying the permalloy transfer path. In this connection, it is noted that the boundary portion 2' has a steep vertical profile like that of the other boundary portion 2. As the consequence, the bubble 1 undergoes a sudden change in energy upon traversing the boundary protion 2', which provides a cause for making difficult smooth transfer of the magnetic bubble from the ion implantation transfer path to the parmalloy transfer path. The same holds for the bubble transfer from the permalloy transfer path to the ion implantation transfer path.